The invention relates to a piezoelectric crystal element with at least one substantially planar surface, to be used in surface acoustic wave devices.
Surface acoustic waves (SAW) on piezoelectric materials are employed for signal processing in numerous components. Uses in the VHF and UHF ranges include filters, oscillators, delay lines, convolvers, and different types of sensors. The major quality criteria for SAW components, which depend on the particular type of substrate used, are temperature stability of delay time or center frequency, low insertion loss (attenuation of a signal by inserting a component into the signal path), and high bandwidth. The latter two criteria are met simultaneously only by strong piezoelectric coupling. The temperature coefficient of the delay time is determined by the difference between the temperature coefficients of phase velocity and path length. If the two effects cancel each other out a temperature-compensated cut will result. Such cuts are obtainable only with a few materials, however.
The principal substrate materials used in this context are ST quartz and lithium niobate. The former is characterized by high temperature stability and weak piezoelectric coupling. Lithium niobate, on the other hand, is characterized by very strong coupling, whereas its delay time strongly increases with temperature. As a compromise between the two requirements lithium tantalate has been used, but its temperature stability has again proved insufficient in a number of applications.
The search for materials combining strong piezoelectric coupling and satisfactory temperature stability has led to such interesting materials as berlinite (ALPO.sub.4) and lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7). Both materials have coupling coefficients higher than that of quartz, in addition to cuts with a negligible first temperature coefficient of delay time. With either material, however, the second (quadratic) temperature coefficient is higher by more than an order of magnitude than that of ST quartz. For this reason neither of the two substrates has become widely accepted.